Natural rubber and its related polyisoprenes, gutta percha and chicle, have many uses and great commercial value. Natural rubber comprises cis-polyisoprene, while gutta percha comprises trans-polyisoprene and chicle is a mixture of the two. Rubber is the most widely used of the polyisoprenes and in many instances no completely successful synthetic substitute has been found. The balance of the present discussion is directed to the recovery of rubber, but it will be understood that the methods of the present invention are equally applicable to the recovery of all three types of polyisoprene. Hereinafter, the word rubber will be used generally to refer to all polyisoprenes that may be so recovered.
Polyisoprenes occur in many plant species in the form of sap, or latex. Table I is a list of some typical rubber, gutta percha, and chicle containing species. Latex is a white, tacky, aqueous suspension of polyisoprene globules encased in protein. The globules are of round or irregular shape, and vary in size from 0.5 to 3.0 microns in diameter.
TABLE I __________________________________________________________________________ Genus and Species Common Name Commercial Method __________________________________________________________________________ Alstonia spp. Multiple incisions Dyera spp. Single incision Funtumia elastica Multiple incisions Landolphia spp. Multiple incisions Cryptostegia grandiflora Cutting of leafless shoots Parthenium argentatum* Guayule Milling of whole plant Scorzonera tau-saghyz* Milling of roots Solidago spp.* Goldenrod Milling or solvent extraction Taraxacum kok-saghyz* Russian dandelion Milling of roots Hevea brasiliensis Para rubber tree Single incision Manihot glaziovii Ceara rubber tree Multiple incisions Castilla (Castilloa) spp. Hule tree Single incision Ficus elastica India or Assam rubber tree Multiple incisions Euonymus europaeus* Spindle tree Solvent extraction Eucommia ulmoides* Solvent extraction Cnidoscolus elasticus* Red chilte Multiple incisions Cnidoscolus tepiquensis* White chilte Multiple incisions Mimusops balata Bullet tree Multiple incisions Palaquium gutta Taban tree Multiple incisions Achras sapota Sapodilla tree Multiple incisions __________________________________________________________________________ *Temperate zone species Dyera spp. are included under rubber producers, but the polymer formed has not been characterized.
At present, natural rubber is most easily recovered from the flowing sap of rubber trees (Hevea brasiliensis), which are indigenous to tropical regions such as Brazil, Malaysia, and Indonesia. Because the latex flows within the tree, it is possible to obtain quantities of latex by simply piercing or cutting the outer bark of the tree and collecting the sap that flows from the cut. If managed properly, latex can be collected from a single tree for years without harm to the tree itself.
For collection of the rubber, latex can be concentrated by evaporation or centrifugation. Coagulation is induced by the addition of acetic acid or formic acid. Once coagulated and collected, the resulting rubber can be chemically treated, such as by vulcanization, and is suitable for many industrial, commercial and household uses.
Besides Hevea, hundreds of other species produce polyisoprenes in varying amounts, including dandelions and milkweed, as illustrated in Table I above. In these other species, the rubber is typically not as accessible as it is in the Hevea plant because it is stored within individual cells in their roots and stems. A few of these species produce commercially significant amounts of rubber, which are currently extracted by expensive and time-consuming processes.
One such species is the guayule plant (Parthenium argentatum), a desert shrub native to parts of Mexico and the southwestern United States. Since the 1940's, guayule has been selectively cultivated so that its rubber content has been greatly increased. One of the most desirable aspects of rubber-production from guayule is the ability of these shrubs to grow on arid land otherwise unsuitable for agriculture. Because guayule accumulates rubber in parenchyma cells in its stems and roots, however, it is necessary to macerate these tissues in order to obtain the rubber contained therein. Thus, the difficulty of recovery of the rubber is still a major obstacle to large-scale commercialization of the guayule rubber industry. Furthermore, guayule plants produce significant amounts of resinous material, which combines with the rubber during extraction and must be removed before the rubber can be put to commercial use.
Presently, rubber is extracted from guayule by one of two methods. The water flotation method consists of crushing and parboiling the guayule plant in water to coagulate the latex. Particles of resin-containing latex float to the surface, along with particles of plant matter. The latex can be collected and washed with acetate to remove most of the resin, following which the rubber is dissolved into hexane and then extracted therefrom by conventional separation methods. This method of rubber recovery is unsuitable in the desert environment in which guayule grows, as it entails enormous energy and water usage, as well as large amounts of organic chemical solvents.
In the second method presently used, rubber is extracted from guayule plants using organic solvents. The extraction can be either sequential or simultaneous. In both cases the guayule plant is typically frozen and pulverized or chipped into flakes. In sequential extraction, resin is removed from the flaked guayule by washing with acetone and the rubber remaining in the flakes is extracted with a nonpolar organic solvent, such as hexane or toluene. In simultaneous extraction, resin-containing rubber is extracted from flaked guayule using a nonpolar solvent. Rubber is then precipitated from the solution using a polar organic solvent such as alcohol.
Neither type of extraction has enjoyed wide-spread commercial success, as the processing steps are time-consuming, expensive, hazardous and energy-intensive. In addition, capital costs for any of the foregoing methods are large, preventing proliferation of local processing plants and therefore necessitating the transport of unprocessed guayule plants over relatively long distances. This is particularly the case where, in underdeveloped countries, industrial processing centers are typically unavailable near areas of production.
Processes are needed which avoid the disadvantages associated with conventional rubber extraction methods and which give a good yield of rubber in a short amount of time. Such processes should achieve the extraction efficiently, without the use of excessive heat or organic solvents. Additionally, such processes will preferably result in a rubber product substantially free of resins and proteins.